1. Field of the Invention
The present invention relates to data recording in a communications system, and, more particularly, to writing information from a channel to a recording medium.
2. Description of the Related Art
Conventional recording systems of the prior art encode data and write the encoded data to a recording medium, such as a magnetic hard drive or an optical recording disk. The encoded data is written to the disk (or other recording medium) by a read/write head (e.g., a magnetic recording/playback head). A read channel component is an, e.g., integrated, circuit of a computer hard disk (HD) drive that encodes, detects, and decodes data, enabling the head to correctly i) write data to the disk drive and ii) read back the data. The disks in an HD drive have a number of tracks, each track consisting of i) user (or “read”) data sectors and ii) control (or “servo”) data sectors embedded between the read sectors. Information stored in the servo sectors is employed to position the head over a track so that the information stored in the read sector can be retrieved properly. Sectors are identified by assigned sector numbers.
In a magnetic recording system, data is recorded by varying the magnetic field over a bit position (“cell”) on the surface of the recording disk. A write bubble is defined as a region within a write-head-induced field where the field intensity is greater than the recording media's coercivity. A transition is formed when the backward expansion of the trailing edge of the write bubble begins to slow down, and eventually is slower than the differential velocity between the head and the medium. The point in time when the bubble expansion rate equals the differential velocity between the head and the medium is the point in time when the transition is formed. The position on the disk of the transition is the location of the trailing edge of the write bubble at that point in time.
FIG. 1A is a diagram illustrating “write bubble” 10 created by write head 12 to magnetically record information on disk 14. See U.S. Pat. No. 6,621,648 B2, the teachings of which are incorporated herein by reference. Write head 12 is configured in a manner well known in the art, and is operable with appropriate write driver circuitry 13 to generate magnetic fields of first and second opposite polarities in response to binary data signals for recording onto disk 14. A data-encoding scheme well known in the art is the Non-Return-to-Zero Inverted (NRZI) encoding scheme, in which a magnetic transition recorded on the disk signifies a binary “one” and the lack of a magnetic transition recorded on the disk signifies a binary “zero.” The region in which the magnetic field is generated is shown as write bubble 10, which is defined as the region in which the magnetic field generated by write head 12 is strong enough to magnetically record on disk 14. Write bubble 10 extends to lateral edges 16a and 16b on disk 14. The tracks of disk 14 move past write head 12 in a direction and at a velocity indicated by the arrow labeled Vmedium. The arrows shown on disk 14 indicate the direction of magnetization of the disk, as recorded by the magnetic field in write bubble 10.
When writing new data over old data on a disk, the magnetization pattern of the new data interacts with the magnetization of the old data. When the leading edge of the write bubble extends into old data, if the polarity of the old data is the same as the polarity being presently induced by the write head, then the transition formed in the media is called an “easy” transition. If, however, the polarity of the old data is opposite that being presently induced by the write head, then the transition formed in the media is called a “hard” transition. Hard transitions are shifted in time relative to easy transitions. These timing shifts can cause degradation in the overall signal-to-noise ratio (SNR) of the magnetic recording system.
FIG. 1B shows a data sector 100 of a prior art magnetic recording system having synchronization (sync) field 101 and user data 102. Sync field 101 is a portion of the data sector, prior to user data 102, that allows the disk drive read channel's phase-lock loop (PLL) to adjust to the desired sample rate to sample points within the disk drive data. The sync field is typically a relatively long interval having a predefined pattern of alternating polarity. A common pattern is an alternating 2T pattern having a period of alternating polarity at two times the minimum bit cell time (i.e., 00110011 . . . , NRZ format).
If a “new” alternating 2T pattern is written over an “old” (i.e., previously recorded) alternating 2T pattern, the typical result is that either i) all of the transitions in the sequence are hard or ii) all of the transitions in the sequence are easy. In either case, when reading the data back, the read clock synchronizes its sampling time (or phase) to one extreme (e.g., the time (or phase) of the hard transitions) or the other extreme (e.g., the time (or phase) of the easy transitions), while the user data that follows the sync field typically has a random distribution of both hard and easy transitions. In such a case, read errors can result from using the skewed read clock derived during the sync field to read data at the start of the user data field.
Prior art methods to reduce the disadvantageous effects of overwriting an alternating 2T pattern with another alternating 2T pattern make the write bubble large enough to minimize interaction of demagnetization fields at the leading and trailing edges of the write bubble. However, as disk drive systems are scaled to smaller sizes, the size of the write bubble doesn't grow. Instead, newer recording systems require smaller write bubbles that make the overwrite problem worse. Also, effects on the recording quality due to the overwrite problem are inversely proportional to the minimum bit cell time (proportional to the data rate).
Another overwrite problem that can exist is when the user data field is overwritten with the same set of data that was previously recorded. As a result of the effect of spindle-speed variations on the phase of written data, the resulting new data can have relatively long intervals of all easy transitions interleaved with relatively long intervals of all hard transitions. During each long interval, the read clock tends to get skewed to the corresponding extreme. This can lead to read errors when moving from an interval of all easy transitions to an interval of all hard transitions, and vice versa.